We purpose to study how spontaneous vasomotor activity influences vascular resistance in the arterial portion of the microcirculation of the dorsal skin fold window chamber implemented in SHR and WKY rats. This preparation allows to investigate the microcirculation without complicating factors due to surgery and anesthesia, which paralyze the microscopic arterial blood vessel. Our objective is to determine whether spontaneous vasomotor activity is different between normotensive and hypertensive subjects, to characterize this differnce, and to deduce what is the corresponding difference in peripheral vascular resistance. We propose to carry out the same type of studies in the human nailfold, and measure the periodic changes of blood flow in the capillaries of the human nailfold which occur as a consequence of the spontaneous vasomotor activity of the feeding arterioles in normotensive and hypertensive subjects. We hypothesize that one of the causes of the increase in peripheral vascular resistance associated with hypertension is, the change in the temporal pattern of spontaneous vasomotor activity. The method of study will consist of : 1) measurement of time dependent diameter and static morphology of the microcirculation in SHR and WKY rats; 2) measurement of capillar blood cell velocity in SHR and WKY rats; 3) measurement of capillary blood cell velocity in the nailfold of normotensive and hypertensive humans; 4) comparison by direct calculations based on Poiseuille's Law of the vascular resistance in WKY and SHR rats and 5) comparison of the power spectrum of capillary blood flow velocity in SHR and WKY rats and normotensive and hypertensive humans. The responsiveness of the vessels from hypertensive subjects will be tested by additional vasodilatory and vasoconstrictive stimuli by measuring the postocclusive reactive hyperemia response and the effect of contralateral cooling. The changes in the responses will be evaluated as a function of the changes in vasomotor patterns and correlated to their effect on peripheral vascular resistance. Subjects will be studied at the time of diagnosis and during therapy.